class Employee extends Party {
    get annualCost() {
        return this.monthlyCost * 12;
    }
}
class Department extends Party {
    get totalAnnualCost() {
        return this.monthlyCost * 12;
    }
}

/******************************************** */
class Party {
    get annualCost() {
        // js 中会自动调用子类上的同名函数
        return this.monthlyCost * 12;
    }

    get monthlyCost() {
        throw new SubclassResponsibilityError();
    }
}